Semi Automatic Ground Environment

Semi Automatic Ground Environment

The Semi-Automatic Ground Environment (SAGE) was an automated control system for tracking and intercepting enemy bomber aircraft used by NORAD from the late 1950s into the 1980s. In later versions, the system could automatically direct aircraft to an interception by sending commands directly to the aircraft's autopilot.

By the time it was fully operational the Soviet bomber threat had been replaced by the Soviet missile threat, for which SAGE was entirely inadequate. Nevertheless, SAGE was tremendously important; it led to huge advances in online systems and interactive computing, real-time computing, and data communications using modems. It is generally considered to be one of the most advanced and successful large computer systems ever developed.

Both MIT and IBM supported the project as contractors. IBM's role in SAGE (the design and manufacture of the AN/FSQ-7 computer, a vacuum tube computer with ferrite core memory based on the never-built Whirlwind II) was an important factor leading to IBM's domination of the computer industry.


Prior to the introduction of SAGE, the task of intercepting bombers was becoming increasingly difficult. This was the latest shift in a balance of power that had been see-sawing since the 1930s.

During the leadup to World War II it was widely believed that the bomber was essentially immune, at least in any practical sense. As speeds approached convert|200|mi/h|km/h|abbr=on the time between seeing the bomber and it reaching its targets was growing so short that there was no time for interceptor aircraft to climb to altitude. Once the bombs were released the multi-engine bombers often had a performance advantage over the fighters, allowing them to escape with relative ease. The only apparent solution to this problem would be to keep fighters in the air on-station at all times, a practical impossibility due to the short flight times of contemporary fighters. Thousands of fighters would be needed to keep enough of them in the air at any one time to defend against a raid of perhaps a hundred bombers. Most believed "the bomber will always get through".

The introduction of radar seriously upset this equation. Radar gave just enough warning time for fighters to "scramble" and be at the bomber's altitude by the time they arrived. In modern terms radar is a "force multiplier", allowing a small number of fighters to handle the task that would otherwise require many more aircraft. As the interception was now being directed by radar screens on the ground, and started at ranges well beyond visual, a system needed to relay interception instructions to the aircraft. Speeds of the aircraft of the era were such that this task could be carried out by hand. The Royal Air Force, for instance, used a large map with markers representing various radar contacts, with controllers relaying positions and directions to the aircraft by radio.

In the post-war era, the speed of the new jet-powered aircraft increased by a factor of two to three, similarly decreasing the available effective response time. In a general sense this should not have caused a problem; although the bombers were approaching much faster and gave less warning time, the fighters intercepting them were also much faster and could climb to altitude in minutes. But it was all of the other tasks that caused the problem. This included collecting information about the targets from the radar sites, figuring out where they were going (developing a "track"), and then guiding the fighters to intercept them. Even the time needed for the pilots to get into the cockpit from their ready rooms nearby became a significant issue as the overall mission times fell. A study in the 1950s by the RCAF concluded that it would take on the order of one minute per interception. With flight times on the order of an hour by several hundred aircraft, some were bound to escape interception due to operator overload. The balance shifted toward the attackers again. With nuclear bombs onboard, this was unacceptable.

The problem became even more acute if the bombers attacked at low level. Radar is line-of-sight, so by approaching close to the ground they would remain hidden behind the curvature of the Earth until approaching to within a few tens of miles. With a jet bomber this meant the defenders had only a few minutes to react, far too little time to launch an interceptor, let alone guide it to an intercept.


It was this problem that particularly bothered Dr. George E. Valley, an MIT physics professor. In order to provide any sort of protection for the entire US, a series of radar stations would have to span both coasts and across Canada. In the event of a raid, there would simply be far too many reports to be able to successfully guide interception. His solution was automation, connecting all of the radar sites to a computer which would then control all of the incoming and outgoing flow of information. The interception operator's workload would be greatly reduced; they simply had to tell the computer which targets to attack, and perhaps choose what assets to use. All of the communications would be handled by the computer, and would be effectively instantaneous.

This would require the system to update the operators in real time, and the only system in the world capable of doing this in 1948 when Valley studied the problem was the Project Whirlwind computer at MIT. The Whirlwind project, originally intended to control a US Navy flight simulator to train bomber crews, had run into problems and the Navy was losing interest. Valley talked to Jay Forrester, leader of the Whirlwind project, and together they wrote a study proposal to use Whirlwind for air defense.

The United States Air Force was interested, and in 1949 they provided funding under the name "Project Charles" to develop a demonstration system. Information from several radars in the Cape Cod area was forwarded to the Whirlwind, which then developed tracks for the targets being reported. The "Cape Cod System" was a qualified success, and the Air Force took over the project under "Project Claude", moving development to the new MIT Lincoln Laboratory in 1954. Making a military-grade version of the Whirlwind was a massive project that required close connections between Lincoln Labs, industrial partners who would build the machines and communications, and the military. In order to provide oversight and management during the deployment phase, MITRE was formed in 1958 to take over the project.

Production of the resulting machines, known technically as the AN/FSQ-7, was initially awarded to RCA but later given to IBM, who started production in 1958. The buildings and internal power supply and communications were provided by Western Electric, phone lines by the Bell System, and the software, 500,000 lines of assembly language, by a spin-off of RAND Corporation called System Development Corporation (SDC).


The AN/FSQ-7 is the largest computer ever built,Fact|date=April 2008 and will likely hold that record in the future. Each machine used 55,000 vacuum tubes, about ½ acre (2,000 m²) of floor space, weighed 275 tons and used up to three megawatts of power. Although the machines used a large number of vacuum tubes, the failure rate of an individual tube was low due to efforts in quality control. Each SAGE site included two computers for redundancy, with one processor on "hot standby" at all times. In spite of the poor reliability of the tubes, this dual-processor design made for remarkably high overall system uptime. 99% availability was not unusual.

SAGE sites were connected to multiple radar stations which transmitted tracking data (range and azimuth) in digitized format by modem over ordinary telephone lines. These digitized inputs were automatically prepared from analog radar inputs by the AN/FST-2B (or successor, AN/FYQ-47 [cite web |url =
title = AN/FYQ-47 Radar Data Processing System |accessdate = 2007-07-18 |publisher = Radomes, Inc.
] ) at the radar stations. The SAGE computers then collected the tracking data for display on a CRT as icons. Situation Display (SD) console operators at the center could select any of the "targets" on the display with a light gun, and then display additional information about the tracking data reported by the radar stations. Up to 150 operators could be supported from each center. Each SD operator console was equipped with an integral cigarette lighter and ashtray.

SAGE site operators could also request height data when needed from their CRT. These height requests were digitized and sent to a radar station that was tracking the "targets". At the radar station, the height requests were displayed to an operator on an analog Range Height Indicator (RHI) CRT display by moving the height cursor. The operator then centered the height cursor on the "target" and depressed a button to send the updated height information back to the SAGE site in much the same way as the tracking data.

When a target was of interest, SAGE also helped the operator to select a proper response. Reports similar to those from the radar stations kept the SAGE system up to date with information on the availability and status of various weapons and aircraft, including all airfields, BOMARC and Nike Hercules anti-aircraft missile sites. When the operator chose one of these to intercept the target, orders would automatically be sent via teletype to local controllers who would take control. Additional messages would also be sent to higher headquarters, as well as other SAGE centers.

In normal operation, communications between the SAGE centers and the interceptor aircraft was relayed via radio equipment at the radar sites, which were more widely spread out than the SAGE centers themselves. A properly equipped aircraft, like the F-106 Delta Dart, could feed the SAGE directions into the autopilot and fly "hands off" to the interception. Older aircraft, which were common when SAGE was first being deployed, could be directed by voice.


A massive building program started along with continued work on the computer systems and communications, with the first groundbreaking at McChord AFB in 1957. The buildings were huge above-ground concrete bricks that were often placed near cities without the residents being aware of what they were. The first SAGE Division became operational in Syracuse, New York in January 1959, and by 1963 the system was already complete with 22 "Sector Direction Centers" and three similar "Combat Centers". When NORAD was set up another site was added at CFB North Bay in Canada, although in this case the entire SAGE system was buried approximately convert|700|ft|m underground in what became known as "the hole".

The total engineering effort for SAGE was immense. Total project cost remains unknown, but estimates place it between 8 and 12 billion 1964 dollars, around 55 billion 2000 dollars, more than the Manhattan Project that developed the nuclear bomb that SAGE defended against.

The SAGE system was operational until 1983, when it was replaced by newer systems and airborne control. The North Bay system ran until 1983 when it was dismantled and sent to "The Computer Museum" in Boston. In 1996 the remainder was moved to Moffett Federal Airfield for storage and is now in the collection of the Computer History Museum in Mountain View, California.

Questions about the ability of the SAGE system to actually handle a "hot war" situation were continuous. On one occasion SAC was able to penetrate the defenses, and on other occasions huge flocks of seabirds were tracked as a potential bomber attack. A more serious problem was that by the time the system was fully operational, the USSR had already started deploying ICBMs, making SAGE largely useless.

To protect against the possibility of SAGE sites being disabled and possibly rendering the defense impotent, the Air Force also developed the Back Up Interceptor Control System (BUIC), a sort of mini-SAGE located at some of the radar sites that normally fed the SAGE system.

In peacetime SAGE was, for all intents, an air traffic control system and it influenced the design of the FAA's automated control systems. The system also gave IBM valuable insight, and it was not long after that the CEO of American Airlines met one of the IBM people involved in SAGE by accident on a flight, and soon the two companies were developing the SABRE airline reservation system.

Other major SAGE developments included:
* CRT-based real-time user interface
* use of wide-area communications via modems
* The installation, operation, and logistic support of over 100 long range radar stations located throughout the US as part of the Air Defense Command

When some of the older SAGE memory units were replaced, they found new life as the computer in Irwin Allen's Time Tunnel.

Further reading

* John F. Jacobs, "The SAGE Air Defense System: A Personal History" (MITRE Corporation, 1986)
* R. G. Enticknap and E. F. Schuster, "SAGE Data System Considerations", "AIEE Transactions" vol 77, pt I, 1958 (January 1959 section), pp 824-832.
* Robert R. Everett (editor), "Special Issue: SAGE (Semi-Automatic Ground Environment)", "Annals of the History of Computing" 5:4 (1983).
* Paul N. Edwards, "The Closed World: Computers and the Politics of Discourse in Cold War America" (Cambridge, MA: MIT Press, 1996) [] , esp. [ Chapter 3] .
* Kent C. Redmond and Thomas M. Smith, "From Whirlwind to MITRE: The R&D Story of The SAGE Air Defense Computer" (Cambridge, MA: MIT Press, 2000) []
* Thomas P. Hughes, "Rescuing Prometheus: Four Monumental Projects That Changed the Modern World" (Pantheon, 1998) [] , esp. Chapter 2.
* Davis Dyer & Michael Aaron Dennis, "Architects of Information Advantage: The MITRE Corporation Since 1958" (Community Communications Corp, December 1998) []

ee also

* Ground-controlled interception (GCI)
* ROTOR, Linesman/Mediator
* DEW Line, Pinetree Line, Mid-Canada Line


External links

* [ Oral history interview] ] with Douglas T. Ross at Charles Babbage Institute, University of Minnesota. Ross recounts his working on MIT's Whirlwind computer in the 1950s. He reports on his first use of Whirlwind for airborne fire control problems. Soon after that the Whirlwind was used for the Cape Cod early warning system, a precursor to the SAGE Air Defense System.
* [ Oral history interview with Severo Ornstein] at Charles Babbage Institute, University of Minnesota. Ornstein describes his experience at Lincoln Laboratory which included work on the SAGE, TX2, and LINC computers.
* [ Oral history interview with Jules I. Schwartz] at Charles Babbage Institute, University of Minnesota. Schwartz worked for the Rand Corporation on SAGE and JOHNNIAC and then at the System Development Corporation. Schwartz details his association with SAGE, and his work on timesharing for the AN/FSQ-32 computer.
* [ Federation of American Scientists - Semi-Automatic Ground Environment (SAGE)]
* [ MITRE History - Semi-Automatic Ground Environment (SAGE)]
* [ MITRE History - Semi-Automatic Ground Environment (SAGE) - Photo Archives]
* [ "On Guard! The Story of SAGE" (12 minute film at the Prelinger Archives)]
* [ SAGE Documents (including aerial photographs, emblems, call signs, etc. of the SAGE sites)]
* [ SAGE computer documents on]
* [ Overview of the Lincoln Laboratory Ballistic Missile Defense Program] (PDF)
* [ Online Air Defense Radar Museum]
* [ SAGE A/N FSQ-7 archive at the Southwest Museum of Engineering,Communications and Computation]
* [ INTRODUCTION TO AN/FSQ-7 COMBAT DIRECTION CENTRAL AND AN/FSQ-8 COMBAT CONTROL CENTRAL] Original IBM "manual" including floor plans and information flow diagrams for a SAGE installation and operation
* [ SAGE Video - Part 1/2]
* [ SAGE Video - Part 2/2]
* [ Records of the System Development Corporation] at Charles Babbage Institute, University of Minnesota. Includes a history file with information about the RAND Corporation, the System Development Division, and the System Development Corporation.

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